This invention relates to electrically conductive buses of the type found in computer systems. More particularly, this invention relates to multiple conductor electrically conductive buses used to electrically intercouple electrical devices found in computer systems.
Computer systems are known which employ electrically conductive bus structures as the mechanism for intercoupling various computer system devices so that the devices are capable of mutual interaction involving the exchange of data and control signals. Many bus systems employ flexible cables, typically flat cables, having embedded therein multiple electrically conductive paths forming the bus, as well as connectors spaced along the cable for enabling electrical interconnection with various devices comprising the computer system, such as host computers, mass storage devices, and file servers. The currently popular SCSI bus defined by ANSI document X3.131, the disclosure of which is hereby incorporated by reference, is an example of such a computer bus. In the SCSI bus, a plurality of devices (typically from two to eight) may be connected, with the individual devices typically consisting of one or two initiators (usually the computer) and one to seven target devices (usually storage devices such as disc drives or tape drives). As with most computer buses, there are technical standards which must be adhered to in order to guarantee reliable signaling among the various devices connected to the bus and thus reliable system operation. For the SCSI bus, the maximum cumulative bus length is limited to 6.0 m. A stub length of no more than 0.1 m is allowed, and it is recommended that stubs be spaced at least 0.3 m apart. For fast synchronous data transfers, the characteristic impedance of the bus must fall within the range of 90-132 ohms. In addition, to minimize discontinuities and signal reflections, the standard recommends that different impedances not be used along the same bus. Also, a terminator is connected to each end of the bus to provide signal integrity and proper voltage level biasing. Other buses have similar standards requiring compliance.
When implementing a computer bus by using cables and connectors, it is at least technically challenging to design and implement a particular bus configuration while ensuring consistency with the bus technical standards. This difficulty is exacerbated by variations in the electrical characteristics of the cables themselves, as well as the specific problems engendered by a particular system configuration. Further, when a plurality of buses are employed in a given computer system, this difficulty is further multiplied. In addition, for a computer system which is intended to be highly configurable, the difficulties in designing a reliable bus structure are further multiplied.